Acoustic feedback occurs because the output loudspeaker signal from an audio system providing amplification of a signal picked up by a microphone is partly returned to the microphone via an acoustic coupling through the air or other media. The part of the loudspeaker signal returned to the microphone is then re-amplified by the system before it is re-presented at the loudspeaker, and again returned to the microphone. As this cycle continues, the effect of acoustic feedback becomes audible as artifacts or even worse, howling, when the system becomes unstable. The problem typically appears when the microphone and the loudspeaker are placed closely together, as e.g. in hearing aids. Some other typical situations with feedback problems are telephony, public address systems, headsets, audio conference systems, etc.
EP 2237573 A1 deals with adaptive feedback cancellation in an audio processing system, e.g. a listening device where specific characteristic properties in an output signal of the forward path are introduced and/or identified. A signal comprising the identified or introduced properties is propagated through the feedback path from output to input transducer and extracted or enhanced on the input side in an Enhancement unit matching (in agreement between the involved units) the introduced and/or identified specific characteristic properties. The signals comprising the specific characteristic properties on the input and output sides, respectively, (i.e. before and after having propagated through the feedback path) are used to estimate the feedback path transfer function in a feedback estimation unit.